To date, a highly concentrated liquid chemical or cleanser has been used in cleaning of substrates serving as electronic parts, surface reforming, or a surface protection process, typically in the RCA clean, and a large amount of pure water or ultrapure water has been used for rinsing the solution or the cleanser. For the purposes of cost reduction in the cleaning process and environmental protection and for other purposes, on the other hand, various simplified cleaning technologies have been developed and accomplished. A typical example of such technologies is a cleaning technology using wash water in which a chemical agent, such as acid or alkali, or a gas, such as ozone or hydrogen, is dissolved in pure water or ultrapure water at a very low concentration. This method reduces the cost of wash water preparation and the cost of waste wash-water disposal as well as reduces, to a large degree, the amount of water used for rinsing after cleaning. This method is also advantageous in that it improves the quality of a substrate surface as it does not involve etching during the cleaning.
The above-described wash water is usually prepared by injecting a cleaning component into a water supply line through which pure water or ultrapure water is supplied from a pure-water manufacturing apparatus or a ultrapure-water manufacturing apparatus, examples of the cleaning component including a chemical agent, such as acid or alkali, and a gas, such as ozone or hydrogen. The prepared wash water is supplied to a use point (use position). In order to achieve a desired cleaning effect, it is important that the concentration of cleaning component in the prepared wash water is constant at a predetermined concentration. Specifically, if the concentration of the cleaning component in the wash water is not constant, the quality of the surfaces of the cleaned electronic parts is affected, decreasing product yields. The injection amount of the cleaning component is thus controlled by controlling means so that wash water containing a cleaning component at a predetermined concentration can be provided, the controlling means acting in conjunction with, for example, a flowmeter provided to a pure/ultrapure water supply line or a cleaning-component concentration sensor provided to a wash-water supply pipe.
Examples of the method for cleaning substrates include batch cleaning and single wafer cleaning. In the batch cleaning, multiple substrates are placed in a special purpose carrier and the substrates in the carrier are collectively cleaned by being immersed in a cleaning bath containing wash water. In the single wafer cleaning, the substrates are cleaned one by one by being placed on a turnable and being sprayed with wash water. In either cleaning, the amount of wash water used by a cleaning device changes in a series of cleaning processes.
Particularly, the single wafer cleaning exerts its cleaning effect by changing the amount of wash water sprayed on substrates or by intermittently spraying wash water in accordance with the cleaning program set on each cleaning device to be used. Thus, the amount of wash water used by the cleaning device changes second by second.
In this manner, in an existing method, wash water is continuously produced at a constant flow rate and directly supplied to a use point while an excess of wash water is expelled in order to stably supply, to the use point, wash water having a constant concentration of cleaning component although the amount of use of wash water changes. In terms of a waste-liquid disposal cost and environmental problems, however, only an amount of wash water required at the use point is desirably supplied without producing excess water that needs disposal.
In order to supply an amount of wash water appropriate for use at the use point, a control mechanism that prepares wash water in accordance with the change in amount of use and supplies the wash water to the use point has to be provided.
A conceivable example of preparing wash water having a constant concentration and supplying the wash water to the use point is as follows. The amount of supply of pure water or ultrapure water is changed in accordance with the amount of use of wash water with the above-described existing method for supplying wash water, while means for controlling the injection amount of cleaning component in conjunction with a flowmeter provided at a water supply line controls the injection amount of cleaning component in accordance with the change in flow rate of pure water or ultrapure water supplied through the water supply line.
However, preparing and supplying wash water having a constant concentration to the use point is difficult by only controlling, in conjunction with the above-described flowmeter, the injection amount of cleaning component to be injected into supply water having a flow rate that changes. Thus, the concentration of the cleaning component in the prepared wash water unavoidably becomes inconstant in accordance with the change in flow rate of supply water. Particularly, when a diluted solution containing a cleaning component at a very low concentration is used as the wash water, the management of the concentration becomes extremely difficult.
An existing method developed to cope with the change in amount of use of wash water is to provide a buffer tank, serving as a container at which wash water stands by before being supplied to the use point, to a wash-water supply pipe so that the buffer tank absorbs the excess of wash water, which is supplied at a constant flow rate, that occurs due to the change in flow rate, or so that the excess of wash water is allowed to be returned upstream from the buffer tank (Patent Literature 1 and 2).
However, electronic parts are cleaned in a highly purified clean room and the equipment occupying the clean room is desired to be space-saving. Thus, providing the buffer tank, the circulation line, and auxiliary equipment for these, as in the cases of Patent Literatures 1 and 2, is not preferable in terms of not only cost but also space saving.